In an image sensing apparatus that employs a solid-state image sensing device such as a CCD or CMOS, the exposure time of the image sensing device is controlled using a mechanical shutter, which is a mechanical light-shield member. To realize a short exposure time, it is necessary to drive the shutter at high speed. This requires mechanical precision and involves difficult control at the same time. Although optical-plane shutters of improved control precision are used in some image sensing apparatus, these are high in cost because they require a complicated mechanism and high precision. Thus, since there are limitations upon exposure-time control with mechanical shutters, electronic-shutter operation is essential in order to realize shutters of higher speed.
An example of an electronic shutter that employs an image sensing device employing an XY-address-type scanning method according to the prior art will now be described.
An electronic shutter operation can be achieved by first executing reset scanning, namely by eliminating unnecessary electric charge, which has accumulated in the pixels of the device, on a per-pixel or per-line basis, and then reading out signal charge upon elapse of a prescribed period of time pixel by pixel or line by line. Such an electronic shutter is referred to as a “rolling electronic shutter”.
The structure of a conventional image sensing device and the operation of a rolling electronic shutter will be described with reference to FIGS. 6 and 7.
FIG. 6 illustrates the structure of an image sensing device that employs an XY-address-type scanning method. The image sensing device includes unit pixels 101. In order to simplify the drawing of FIG. 6, only a 4×4 array of the unit pixels 101 is illustrated. In actuality, however, a very large number of such unit pixels are arrayed in two dimensions. Each unit pixel 101 includes a photodiode 102 for converting light to electric charge; a floating diffusion area 104 that temporarily stores electric charge; a transfer switch 103 for transferring electric charge, which has been produced by the photodiode 102, to the floating diffusion area 104 by a transfer pulse φTX; a MOS amplifier 105 that functions as a source follower; a selection switch 106 for selecting the pixel in response to a selection pulse φSEL; and a reset switch 107 for eliminating electric charge, which has accumulated in the floating diffusion area 104, in response to a reset pulse φRES. A floating depletion amplifier is constructed by the floating diffusion area 104, MOS amplifier 105 and a constant-current source 109, which is described later. Signal charge in the pixel selected by the selection switch 106 is converted to voltage and the voltage is output to a read-out circuit 113 via a signal output line 108. The constant-current source 109 has the MOS amplifier 105 as its load. A selection switch 110, which is for selecting an output signal from the read-out circuit 113, is driven by a horizontal scanning circuit 114. An output amplifier 111 is for outputting a signal to the outside of the image sensing device. A vertical scanning circuit 112 is for selecting the switches 103, 106 and 107.
Pulse signals applied to, e.g., an nth scanning line scanned and selected by the vertical scanning circuit 112 are denoted φTXn, φRESn and φSELn with regard to the pulse signals φTX, φRES and φSEL, respectively.
FIG. 7 illustrates driving pulses and the operating sequence relating to the operation of the rolling electronic shutter. In order to simplify the description regarding FIG. 7, the description rendered below will relate to control for driving four lines, namely line n to line n+3, scanned and selected by the vertical scanning circuit 112.
First, from time t31 to time t32 in regard to line n, a reset operation is performed by applying the pulse signals φRESn and φTXn, thereby turning on the transfer switches 103 and reset switches 107 to eliminate unnecessary electric charge that has accumulated in the photodiodes 102 and floating diffusion areas 104 of the nth line. A charge accumulating operation, in which the transfer switches 103 are turned off and photoelectric charge produced by the photodiodes 102 is accumulated, starts at time t32. Next, at time t34, a transfer operation is performed by applying the pulse φTXn, thereby turning on the transfer switches 103 to transfer photoelectric charge, which has accumulated in the photodiodes 102, to the floating diffusion areas 104. It should be noted that it is necessary to turn off the reset switches 107 in advance of the transfer operation. In control of drive illustrated in FIG. 7, the reset switches 107 are turned off at the same time as the transfer switches 103 are turned off at time t32. Accordingly, charge accumulation time is from time t32, at which the reset operation ends, to time t35, at which transfer ends.
By applying the pulse φSEL to turn on the selection switches 106 following the end of the transfer operation for the nth line, electric charge held in the floating diffusion areas 104 is converted to voltage and the voltage is output to the read-out circuit 113. Signals held temporarily by the read-out circuit 113 are output successively by the horizontal scanning circuit 114 at time t36. Let T3read represent the period of time from the start of transfer at time t34 to the end of read-out at time t37, and let T3wait represent the period of time from t31 to t33. Similarly, with regard to other lines, T3read is the period of time from the start of transfer to the end of read-out, and T3wait is the period of time from start of reset of a certain line to the start of reset of the next line.
There is a MOS-type image sensing device that performs a batch electronic shutter operation, in which the reset operations are performed all at once and the read-out operations are performed all at once. This operating sequence is illustrated in FIG. 8.
As shown in FIG. 8, reset operations for all lines are executed simultaneously from time t41 to time t42, and transfer operations are carried out simultaneously from time t43 to time t44. Such an electronic shutter is referred to as a “batch electronic shutter”. When a batch electronic shutter is implemented, the accumulation time period is t42 to t44 for all lines, and the accumulation timings can be made the same from the top to the bottom of the screen (for example, see the specification of Japanese Patent Application Laid-Open No. 2003-17677).
A problem with the operation of the rolling electronic shutter of the prior art described above is that the accumulation timings at the top and bottom of the screen differ by the length of time necessary for scanning of the screen. This is because T3wait, which is the length of time from reset and transfer scan of a certain line to reset and transfer scan of the next line, is required to be greater than the time period T3read from start of transfer to end of read-out. The reason for this is that if T3wait is shorter than T3read, the image signal of the next line is output to the read-out circuit before the read-out operation of the present line is completed, as a result of which accurate image information cannot be obtained. Accordingly, read-out and scanning of image signals cannot be performed at high speed and a large disparity develops between the accumulation timings at the top and bottom of the screen, especially in a case where the screen is composed of a large number of pixels.
In the conventional batch electronic shutter operation described above, the reset and transfer operations are such that selection is performed collectively, and the read-out operation is such that scanning is performed line by line. This means that a circuit for implementing a batch selection operation is required anew. This invites a rise in cost, which accompanies an increase in the surface area of the semiconductor chip.
In a case where a still picture is taken using an electronic shutter, often the accumulation of charge is started after each pixel is cleared by the electronic shutter, and light is blocked by a mechanical shutter upon elapse of a prescribed period of time, thereby terminating charge accumulation, which is followed by read-out, as described above. The reason for this is that in a case where the CCD used as the image sensing device is a frame-transfer CCD or an interline two-field readout CCD, it is necessary that read-out of electric charge be performed in a state in which light is blocked. Further, even in the case of other CCDs, it is necessary to perform read-out upon blocking light to the CCD in order to achieve an improvement in image quality by positively preventing smear (a phenomenon in which electric charge that has overflowed from CCD pixels flows into the transfer portion and light appears so as to produce streaks along the vertical direction of the screen).
Further, if use is made of a MOS-type image sensing device, photoelectric charge will be produced in the floating diffusion area in a case where the image sensing device has such a structure that light leaks into the floating diffusion area. This means that the light must be blocked by a mechanical shutter after the transfer of charge to the floating diffusion area. With a mechanical shutter such as a focal-plane shutter and a batch electronic shutter, shutter scanning time of the batch electronic shutter, which decides the start of exposure, differs from the shutter scanning time of the mechanical shutter, which decides the end of exposure. As a consequence, exposure unevenness develops in the scanning direction of the mechanical shutter and this results in a decline in image quality.